This invention relates to rotor constructions in turbo-machinery, and specifically, to an aerodynamic scoop for a rotor sub-slot entrance in a gas turbine rotor assembly.
Turbine rotors are formed with an array of radial slots that extend axially along substantially the entire length of the rotor. These slots are designed to receive a plurality of copper turns, stacked one upon the other. Ventilation circuits are provided for cooling the rotor and the copper turns, and the ventilation arrangement may include a sub-slot at the base of each of the radial slots in which the copper turns are seated. An elongated cover separates the end turns from the sub-slot, allowing ventilating air to enter into the sub-slot and circulate through a plurality of passages formed in the cover and within the copper turns themselves.
It has been discovered that the largest consumer of shaft power in the rotor ventilation circuit is the loss that occurs at the rotor sub-slot entrances. Much of the loss is associated with the acceleration of the coolant flow as it enters the sub-slot, and is difficult to avoid. A large part of the loss, however, is associated with the inlet condition of the sub-slot. It has long been known that the inlet edges of the slot must be machined to provide a gradual radius. Nevertheless, the cost of machining the inlet edges is quite high, and involves many man hours of effort. In addition, variation of edge configuration from slot to slot is high. Thus, there is a need for a low cost, aerodynamically xe2x80x9cperfectxe2x80x9d sub-slot entrance condition.
In accordance with an exemplary embodiment of this invention, molded plastic aerodynamic scoops are located at the rotor sub-slot entrances to provide a significantly improved aerodynamically sub-slot entrance condition. Preferably, the scoops are molded out of a material such as plastic, but other materials may also be suitable. The walls of the scoops must be very thin, however, especially if the scoops take up sub-slot space. It is expected that the improvement in inlet lost losses, however, will offset any increase in the acceleration losses due to decreased sub-slot space.
The aerodynamic scoop can be attached to the sub-slot cover to restrain against axial migration and, if desired, can be at least partially dovetailed to the sub-slot walls to provide radial restraint against centrifugal forces. Of course, the copper turns in the radial slots also provide radial restraint.
In the exemplary embodiment, side walls of the rotor sub-slot scoop in accordance with this invention flare outwardly from the sub-slot entrance in a lateral or horizontal direction (as opposed to a vertical or radial direction) such that the cooling air is funneled into the sub-slot. Top and bottom (radially outer and inner) surfaces of the scoop are preferably flat due to upper and lower interferences with copper turns and rotor spindles.
Accordingly, in its broader aspects, the present invention relates to a rotor for a turbine comprising a plurality of radially oriented, axially extending slots, each slot having a plurality of field windings seated therein; a sub-slot cover separating said windings from a base of the slot, thereby forming a coolant sub-slot radially inwardly of the windings; and an aerodynamic scoop fixed at an entrance to the sub-slot.
In another aspect, the invention relates to an aerodynamic scoop for attachment to a sub-slot in a gas turbine rotor assembly, the scoop comprising a sub-slot end having a first width substantially equal to a corresponding width of the sub-slot; a flow entrance end having a second width greater than the first width; and sidewalls curving laterally outwardly from the sub-slot end to the flow entrance end.